Examples of the multicellular biodiversity of the Earth.
Global biodiversity is the measure of biodiversity on planet Earth and is defined as the total variability of life forms. More than 99 percent of all species that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 2 million to 1 trillion, of which about 1.74 million have been databased thus far and over 80 percent have not yet been described.
More recently, in May 2016, scientists reported that 1 trillion species
are estimated to be on Earth currently with only one-thousandth of one
percent described. The total amount of DNAbase pairs on Earth, as a possible approximation of global biodiversity, is estimated at 5.0 x 1037, and weighs 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).
In other related studies, around 1.9 million extinct species are believed to have been described currently,
but some scientists believe 20% are synonyms, reducing the total valid
described species to 1.5 million. In 2013, a study published in Science estimated there to be 5 ± 3 million extant species on Earth. Another study, published in 2011 by PLoS Biology, estimated there to be 8.7 million ± 1.3 million eukaryotic species on Earth. Some 250,000 valid fossil species have been described, but this is believed to be a small proportion of all species that have ever lived.
Global biodiversity is affected by extinction and speciation. The background extinction rate
varies among taxa but it is estimated that there is approximately one
extinction per million species years. Mammal species, for example,
typically persist for 1 million years. Biodiversity has grown and shrunk
in earth's past due to (presumably) abiotic factors such as extinction events
caused by geologically rapid changes in climate. Climate change 299
million years ago was one such event. A cooling and drying resulted in
catastrophic rainforest collapse and subsequently a great loss of
diversity, especially of amphibians.
Biodiversity
is usually plotted as the richness of a geographic area, with some
reference to a temporal scale. Types of biodiversity include taxonomic or species, ecological, morphological, and genetic diversity. Taxonomic diversity, that is the number of species, genera, family is the most commonly assessed type.
A few studies have attempted to quantitatively clarify the relationship
between different types of diversity. For example, the biologist Sarda
Sahney has found a close link between vertebrate taxonomic and
ecological diversity.
Known species
Insects make up the vast majority of animal species.
Chapman, 2005 and 2009
has attempted to compile perhaps the most comprehensive recent
statistics on numbers of extant species, drawing on a range of published
and unpublished sources, and has come up with a figure of approximately
1.9 million estimated described taxa, as against possibly a total of
between 11 and 12 million anticipated species overall (described plus
undescribed), though other reported values for the latter vary widely.
It is important to note that in many cases, the values given for
"Described" species are an estimate only (sometimes a mean of reported
figures in the literature) since for many of the larger groups in
particular, comprehensive lists of valid species names do not currently
exist. For fossil species, exact or even approximate numbers are harder
to find; Raup, 1986
includes data based on a compilation of 250,000 fossil species so the
true number is undoubtedly somewhat higher than this. It should also be
noted that the number of described species is increasing by around
18,000–19,000 extant, and approaching 2,000 fossil species each year, as
of 2012. The number of published species names
is higher than the number of described species, sometimes considerably
so, on account of the publication, through time, of multiple names (synonyms) for the same accepted taxon in many cases.
Based on Chapman's (2009) report, the estimated numbers of described extant species as of 2009 can be broken down as follows:
In 1982, Terry Erwin
published an estimate of global species richness of 30 million, by
extrapolating from the numbers of beetles found in a species of tropical
tree. In one species of tree, Erwin identified 1200 beetle species, of
which he estimated 163 were found only in that type of tree.
Given the 50,000 described tropical tree species, Erwin suggested that
there are almost 10 million beetle species in the tropics. In 2011 a study published in PLoS Biology estimated there to be 8.7 million ± 1.3 million eukaryotic species on Earth. A 2016 study concludes that Earth is home to 1 trillion species.
Indices
After the Convention on Biological Diversity
was signed in 1992, biological conservation became a priority for the
international community. There are several indicators used that describe
trends in global biodiversity. However, there is no single indicator
for all extant species as not all have been described and measured over
time. There are different ways to measure changes in biodiversity. The Living Planet Index
(LPI) is a population-based indicator that combines data from
individual populations of many vertebrate species to create a single
index.
The Global LPI for 2012 decreased by 28%. There are also indices that
separate temperate and tropical species for marine and terrestrial
species. The Red List Index is based on the IUCN Red List
of Threatened Species and measures changes in conservation status over
time and currently includes taxa that have been completely categorized:
mammals, birds, amphibians and corals.
The Global Wild Bird Index is another indicator that shows trends in
population of wild bird groups on a regional scale from data collected
in formal surveys. Challenges to these indices due to data availability are taxonomic gaps and the length of time of each index.
The Biodiversity Indicators Partnership
was established in 2006 to assist biodiversity indicator development,
advancement and to increase the availability of indicators.
Importance of biodiversity
Biodiversity is important for humans through ecosystem services
and goods. Ecosystem services are broken down into: regulating services
such as air and water purification, provisioning services (goods), such
as fuel and food, cultural services and supporting services such as
pollination and nutrient cycling.
Biodiversity loss
Summary
of major biodiversity-related environmental-change categories expressed
as a percentage of human-driven change (in red) relative to baseline
(blue)
Even though permanent global species loss is a more dramatic and tragic phenomenon than regional changes in species composition, even minor changes from a healthy stable state can have dramatic influence on the food web and the food chain insofar as reductions in only one species can adversely affect the entire chain (coextinction), leading to an overall reduction in biodiversity, possible alternative stable states of an ecosystem notwithstanding. Ecological effects of biodiversity are usually counteracted by its loss. Reduced biodiversity in particular leads to reduced ecosystem services and eventually poses an immediate danger for food security, but also can have more lasting public health consequences for humans.
International environmental organizations have been campaigning to
prevent biodiversity loss for decades, public health officials have
integrated it into the One Health
approach to public health practice, and increasingly preservation of
biodiversity is part of international policy. For example, the UN Convention on Biological Diversity
is focused on preventing biodiversity loss and proactive conservation
of wild areas. The international commitment and goals for this work is
currently embodied by Sustainable Development Goal 15 "Life on Land" and Sustainable Development Goal 14 "Life Below Water". However, the United Nations Environment Programme
report on "Making Peace with Nature" released in 2020 found that most
of these efforts had failed to meet their international goals.
The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. However, only about 14% of these had been described by 2011.
While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example, the boundaries between closely related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, and in a ring species. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies.
Although none of these are entirely satisfactory definitions, and while
the concept of species may not be a perfect model of life, it is still
an incredibly useful tool to scientists and conservationists
for studying life on Earth, regardless of the theoretical difficulties.
If species were fixed and clearly distinct from one another, there
would be no problem, but evolutionary processes cause species to change continually, and to grade into one another.
Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics.
Early taxonomists such as Linnaeus had no option but to describe what
they saw: this was later formalised as the typological or morphological
species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or even impossible to test. Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others.
Many of the concepts are quite similar or overlap, so they are not easy
to count: the biologist R. L. Mayden recorded about 24 concepts, and the philosopher of science John Wilkins counted 26.
Wilkins further grouped the species concepts into seven basic kinds of
concepts: (1) agamospecies for asexual organisms (2) biospecies for
reproductively isolated sexual organisms (3) ecospecies based on
ecological niches (4) evolutionary species based on lineage (5) genetic
species based on gene pool (6) morphospecies based on form or phenotype
and (7) taxonomic species, a species as determined by a taxonomist.
Typological or morphological species
All adult Eurasian blue tits share the same coloration, unmistakably identifying the morphospecies.
A typological species is a group of organisms in which individuals
conform to certain fixed properties (a type), so that even pre-literate
people often recognise the same taxon as do modern taxonomists.
The clusters of variations or phenotypes within specimens (such as
longer or shorter tails) would differentiate the species. This method
was used as a "classical" method of determining species, such as with
Linnaeus early in evolutionary theory. However, different phenotypes are
not necessarily different species (e.g. a four-winged Drosophila born to a two-winged mother is not a different species). Species named in this manner are called morphospecies.
In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on the morphological species concept, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms.
It differs from the morphological species concept in including a
numerical measure of distance or similarity to cluster entities based on
multivariate comparisons of a reasonably large number of phenotypic
traits.
Recognition and cohesion species
A mate-recognition species is a group of sexually reproducing organisms that recognise one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion
species is the most inclusive population of individuals having the
potential for phenotypic cohesion through intrinsic cohesion mechanisms;
no matter whether populations can hybridise successfully, they are
still distinct cohesion species if the amount of hybridisation is
insufficient to completely mix their respective gene pools. A further development of the recognition concept is provided by the biosemiotic concept of species.
In microbiology,
genes can move freely even between distantly related bacteria, possibly
extending to the whole bacterial domain. As a rule of thumb,
microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA hybridisation to decide if they belong to the same species or not. (This concept was narrowed in 2006 to a similarity of 98.7%.
DNA-DNA hybridisation is outdated, and results have sometimes led to misleading conclusions about species, as with the pomarine and great skua. Modern approaches compare sequence similarity using computational methods.
DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use. One of the barcodes is a region of mitochondrial DNA within the gene for cytochrome c oxidase. A database, Barcode of Life Data Systems (BOLD), contains DNA barcode sequences from over 190,000 species.
However, scientists such as Rob DeSalle have expressed concern that
classical taxonomy and DNA barcoding, which they consider a misnomer,
need to be reconciled, as they delimit species differently.
Genetic introgression mediated by endosymbionts and other vectors can
further make barcodes ineffective in the identification of species.
Phylogenetic or cladistic species
The
cladistic or phylogenetic species concept is that a species is the
smallest lineage which is distinguished by a unique set of either
genetic or morphological traits. No claim is made about reproductive
isolation, making the concept useful also in palaeontology where only
fossil evidence is available.
A phylogenetic or cladistic
species is "the smallest aggregation of populations (sexual) or
lineages (asexual) diagnosable by a unique combination of character
states in comparable individuals (semaphoronts)".
The empirical basis – observed character states – provides the evidence
to support hypotheses about evolutionarily divergent lineages that have
maintained their hereditary integrity through time and space. Molecular markers may be used to determine diagnostic genetic differences in the nuclear or mitochondrial DNA of various species. For example, in a study done on fungi,
studying the nucleotide characters using cladistic species produced the
most accurate results in recognising the numerous fungi species of all
the concepts studied. Versions of the phylogenetic species concept that emphasise monophyly or diagnosability may lead to splitting of existing species, for example in Bovidae,
by recognising old subspecies as species, despite the fact that there
are no reproductive barriers, and populations may intergrade
morphologically. Others have called this approach taxonomic inflation, diluting the species concept and making taxonomy unstable.
Yet others defend this approach, considering "taxonomic inflation"
pejorative and labelling the opposing view as "taxonomic conservatism";
claiming it is politically expedient to split species and recognise
smaller populations at the species level, because this means they can
more easily be included as endangered in the IUCNred list and can attract conservation legislation and funding.
Unlike the biological species concept, a cladistic species does
not rely on reproductive isolation – its criteria are independent of
processes that are integral in other concepts. Therefore, it applies to asexual lineages.
However, it does not always provide clear cut and intuitively
satisfying boundaries between taxa, and may require multiple sources of
evidence, such as more than one polymorphic locus, to give plausible
results.
Evolutionary species
An evolutionary species, suggested by George Gaylord Simpson
in 1951, is "an entity composed of organisms which maintains its
identity from other such entities through time and over space, and which
has its own independent evolutionary fate and historical tendencies".
This differs from the biological species concept in embodying
persistence over time. Wiley and Mayden stated that they see the
evolutionary species concept as "identical" to Willi Hennig's
species-as-lineages concept, and asserted that the biological species
concept, "the several versions" of the phylogenetic species concept, and
the idea that species are of the same kind as higher taxa are not
suitable for biodiversity studies (with the intention of estimating the
number of species accurately). They further suggested that the concept
works for both asexual and sexually-reproducing species. A version of the concept is Kevin de Queiroz's "General Lineage Concept of Species".
Ecological species
An
ecological species is a set of organisms adapted to a particular set of
resources, called a niche, in the environment. According to this
concept, populations form the discrete phenetic clusters that we
recognise as species because the ecological and evolutionary processes
controlling how resources are divided up tend to produce those clusters.
Genetic species
A
genetic species as defined by Robert Baker and Robert Bradley is a set
of genetically isolated interbreeding populations. This is similar to
Mayr's Biological Species Concept, but stresses genetic rather than
reproductive isolation.
In the 21st century, a genetic species can be established by comparing
DNA sequences, but other methods were available earlier, such as
comparing karyotypes (sets of chromosomes) and allozymes (enzyme variants).
Evolutionarily significant unit
An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation.
Chronospecies
A chronospecies is defined in a single lineage (solid line) whose morphology
changes with time. At some point, palaeontologists judge that enough
change has occurred that two species (A and B), separated in time and
anatomy, once existed.
In palaeontology, with only comparative anatomy (morphology) from fossils as evidence, the concept of a chronospecies can be applied. During anagenesis
(evolution, not necessarily involving branching), palaeontologists seek
to identify a sequence of species, each one derived from the phyletically extinct
one before through continuous, slow and more or less uniform change. In
such a time sequence, palaeontologists assess how much change is
required for a morphologically distinct form to be considered a
different species from its ancestors.
Viruses have enormous populations, are doubtfully living since they
consist of little more than a string of DNA or RNA in a protein coat,
and mutate rapidly. All of these factors make conventional species
concepts largely inapplicable. A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly mutagenic environment, and hence governed by a mutation–selection balance. It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape
will outcompete a quasispecies located at a higher but narrower fitness
peak in which the surrounding mutants are unfit, "the quasispecies
effect" or the "survival of the flattest". There is no suggestion that a
viral quasispecies resembles a traditional biological species.
Taxonomy and naming
A cougar, mountain lion, panther, or puma, among other common names: its scientific name is Puma concolor.
Common and scientific names
The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, Felis catus, or the cat family, Felidae.
Another problem with common names is that they often vary from place to
place, so that puma, cougar, catamount, panther, painter and mountain
lion all mean Puma concolor in various parts of America, while "panther" may also mean the jaguar (Panthera onca) of Latin America or the leopard (Panthera pardus) of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal; they are in two parts used together: the genus as in Puma, and the specific epithet as in concolor.
A species is given a taxonomic name when a type specimen
is described formally, in a publication that assigns it a unique
scientific name. The description typically provides means for
identifying the new species, differentiating it from other previously
described and related or confusable species and provides a validly published name (in botany) or an available name
(in zoology) when the paper is accepted for publication. The type
material is usually held in a permanent repository, often the research
collection of a major museum or university, that allows independent
verification and the means to compare specimens. Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature, are "appropriate, compact, euphonious, memorable, and do not cause offence".
Abbreviations
Books
and articles sometimes intentionally do not identify species fully,
using the abbreviation "sp." in the singular or "spp." (standing for species pluralis, the Latin for multiple species) in the plural in place of the specific name or epithet (e.g. Canis
sp.). This commonly occurs when authors are confident that some
individuals belong to a particular genus but are not sure to which exact
species they belong, as is common in paleontology.
Authors may also use "spp." as a short way of saying that
something applies to many species within a genus, but not to all. If
scientists mean that something applies to all species within a genus,
they use the genus name without the specific name or epithet. The names
of genera and species are usually printed in italics. However, abbreviations such as "sp." should not be italicised.
When a species's identity is not clear, a specialist may use
"cf." before the epithet to indicate that confirmation is required. The
abbreviations "nr." (near) or "aff." (affine) may be used when the
identity is unclear but when the species appears to be similar to the
species mentioned after.
Identification codes
With
the rise of online databases, codes have been devised to provide
identifiers for species that are already defined, including:
Kyoto Encyclopedia of Genes and Genomes (KEGG) employs a three- or four-letter code for a limited number of organisms; in this code, for example, H. sapiens is simply hsa.
UniProt employs an "organism mnemonic" of not more than five alphanumeric characters, e.g., HUMAN for H. sapiens.
The naming of a particular species, including which genus (and higher taxa) it is placed in, is a hypothesis
about the evolutionary relationships and distinguishability of that
group of organisms. As further information comes to hand, the hypothesis
may be corroborated or refuted. Sometimes, especially in the past when
communication was more difficult, taxonomists working in isolation have
given two distinct names to individual organisms later identified as the
same species. When two species names are discovered to apply to the
same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymy. Dividing a taxon into multiple, often new, taxa is called splitting.
Taxonomists are often referred to as "lumpers" or "splitters" by their
colleagues, depending on their personal approach to recognising
differences or commonalities between organisms.
The circumscription of taxa, considered a taxonomic decision at the
discretion of cognizant specialists, is not governed by the Codes of
Zoological or Botanical Nomenclature.
The nomenclatural codes that guide the naming of species, including the ICZN for animals and the ICN
for plants, do not make rules for defining the boundaries of the
species. Research can change the boundaries, also known as
circumscription, based on new evidence. Species may then need to be
distinguished by the boundary definitions used, and in such cases the
names may be qualified with sensu stricto
("in the narrow sense") to denote usage in the exact meaning given by
an author such as the person who named the species, while the antonymsensu lato
("in the broad sense") denotes a wider usage, for instance including
other subspecies. Other abbreviations such as "auct." ("author"), and
qualifiers such as "non" ("not") may be used to further clarify the
sense in which the specified authors delineated or described the
species.
Most modern textbooks make use of Ernst Mayr's 1942 definition, known as the Biological Species Concept
as a basis for further discussion on the definition of species. It is
also called a reproductive or isolation concept. This defines a species
as
groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.
It has been argued that this definition is a natural consequence of
the effect of sexual reproduction on the dynamics of natural selection.
Mayr's use of the adjective "potentially" has been a point of debate;
some interpretations exclude unusual or artificial matings that occur
only in captivity, or that involve animals capable of mating but that do
not normally do so in the wild.
It is difficult to define a species in a way that applies to all organisms. The debate about species delimitation is called the species problem. The problem was recognised even in 1859, when Darwin wrote in On the Origin of Species:
No one definition has satisfied all
naturalists; yet every naturalist knows vaguely what he means when he
speaks of a species. Generally the term includes the unknown element of a
distinct act of creation.
When Mayr's concept breaks down
Palaeontologists are limited to morphological evidence when deciding whether fossil life-forms like these Inoceramus bivalves formed a separate species.
A simple textbook definition, following Mayr's concept, works well for most multi-celled organisms, but breaks down in several situations:
When scientists do not know whether two morphologically similar
groups of organisms are capable of interbreeding; this is the case with
all extinct life-forms in palaeontology, as breeding experiments are not possible.
When hybridisation permits substantial gene flow between species.
In ring species,
when members of adjacent populations in a widely continuous
distribution range interbreed successfully but members of more distant
populations do not.
Species identification is made difficult by discordance between
molecular and morphological investigations; these can be categorised as
two types: (i) one morphology, multiple lineages (e.g. morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages). In addition, horizontal gene transfer (HGT) makes it difficult to define a species.
All species definitions assume that an organism acquires its genes from
one or two parents very like the "daughter" organism, but that is not
what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes, including some crustaceans and echinoderms.
The evolutionary biologist James Mallet concludes that
there is no easy way to tell
whether related geographic or temporal forms belong to the same or
different species. Species gaps can be verified only locally and at a
point of time. One is forced to admit that Darwin's insight is correct:
any local reality or integrity of species is greatly reduced over large
geographic ranges and time periods.
The species concept is further weakened by the existence of microspecies, groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates. For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion, complicated by hybridisation, apomixis and polyploidy, making gene flow between populations difficult to determine, and their taxonomy debatable. Species complexes occur in insects such as Heliconius butterflies, vertebrates such as Hypsiboas treefrogs, and fungi such as the fly agaric.
Natural hybridisation
presents a challenge to the concept of a reproductively isolated
species, as fertile hybrids permit gene flow between two populations.
For example, the carrion crowCorvus corone and the hooded crowCorvus cornix appear and are classified as separate species, yet they hybridise freely where their geographical ranges overlap.
Hybridisation of carrion and hooded crows permits gene flow between 'species'
A ring species
is a connected series of neighbouring populations, each of which can
sexually interbreed with adjacent related populations, but for which
there exist at least two "end" populations in the series, which are too
distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing the ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare. Proposed examples include the herring gull-lesser black-backed gull complex around the North pole, the Ensatina eschscholtzii group of 19 populations of salamanders in America, and the greenish warbler in Asia,
but many so-called ring species have turned out to be the result of
misclassification leading to questions on whether there really are any
ring species.
Seven "species" of Larus gulls interbreed in a ring around the Arctic.
Opposite ends of the ring: a herring gull (Larus argentatus) (front) and a lesser black-backed gull (Larus fuscus) in Norway
Presumed evolution of five "species" of greenish warblers around the Himalayas
Change
Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct.
The evolutionary process by which biological populations evolve to become distinct or reproductively isolated as species is called speciation. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species. Speciation depends on a measure of reproductive isolation, a reduced gene flow. This occurs most easily in allopatric
speciation, where populations are separated geographically and can
diverge gradually as mutations accumulate. Reproductive isolation is
threatened by hybridisation, but this can be selected against once a
pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model.
A different mechanism, phyletic speciation, involves one lineage
gradually changing over time into a new and distinct form, without
increasing the number of resultant species.
Horizontal gene transfer between organisms of different species, either through hybridisation, antigenic shift, or reassortment,
is sometimes an important source of genetic variation. Viruses can
transfer genes between species. Bacteria can exchange plasmids with
bacteria of other species, including some apparently distantly related
ones in different phylogenetic domains, making analysis of their relationships difficult, and weakening the concept of a bacterial species.
Louis-Marie Bobay and Howard Ochman suggest, based on analysis of
the genomes of many types of bacteria, that they can often be grouped
"into communities that regularly swap genes", in much the same way that
plants and animals can be grouped into reproductively isolated breeding
populations. Bacteria may thus form species, analogous to Mayr's
biological species concept, consisting of asexually reproducing
populations that exchange genes by homologous recombination.
A species is extinct when the last individual of that species dies, but it may be functionally extinct
well before that moment. It is estimated that over 99 percent of all
species that ever lived on Earth, some five billion species, are now
extinct. Some of these were in mass extinctions such as those at the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods. Mass extinctions had a variety of causes including volcanic activity, climate change,
and changes in oceanic and atmospheric chemistry, and they in turn had
major effects on Earth's ecology, atmosphere, land surface and waters. Another form of extinction is through the assimilation of one species
by another through hybridization. The resulting single species has been
termed as a "compilospecies".
Practical implications
Biologists and conservationists
need to categorise and identify organisms in the course of their work.
Difficulty assigning organisms reliably to a species constitutes a
threat to the validity of research results, for example making measurements of how abundant a species is in an ecosystem
moot. Surveys using a phylogenetic species concept reported 48% more
species and accordingly smaller populations and ranges than those using
nonphylogenetic concepts; this was termed "taxonomic inflation",
which could cause a false appearance of change to the number of
endangered species and consequent political and practical difficulties.
Some observers claim that there is an inherent conflict between the
desire to understand the processes of speciation and the need to
identify and to categorise.
Conservation laws in many countries make special provisions to
prevent species from going extinct. Hybridization zones between two
species, one that is protected and one that is not, have sometimes led
to conflicts between lawmakers, land owners and conservationists. One of
the classic cases in North America is that of the protected northern spotted owl which hybridises with the unprotected California spotted owl and the barred owl; this has led to legal debates.
It has been argued that the species problem is created by the varied
uses of the concept of species, and that the solution is to abandon it
and all other taxonomic ranks, and use unranked monophyletic groups
instead. It has been argued, too, that since species are not comparable,
counting them is not a valid measure of biodiversity; alternative measures of phylogenetic biodiversity have been proposed.
In his biology, Aristotle used the term γένος (génos) to mean a kind, such as a bird or fish, and εἶδος (eidos) to mean a specific form within a kind, such as (within the birds) the crane, eagle, crow, or sparrow. These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean terms thus named; today the birds are a class, the cranes are a family, and the crows a genus. A kind was distinguished by its attributes;
for instance, a bird has feathers, a beak, wings, a hard-shelled egg,
and warm blood. A form was distinguished by being shared by all its
members, the young inheriting any variations they might have from their
parents. Aristotle believed all kinds and forms to be distinct and
unchanging. His approach remained influential until the Renaissance.
Fixed species
John Ray believed that species breed true and do not change, even though variations exist.
When observers in the Early Modern
period began to develop systems of organization for living things, they
placed each kind of animal or plant into a context. Many of these early
delineation schemes would now be considered whimsical: schemes included
consanguinity based on colour (all plants with yellow flowers) or
behaviour (snakes, scorpions and certain biting ants). John Ray, an English naturalist, was the first to attempt a biological definition of species in 1686, as follows:
No surer criterion for determining
species has occurred to me than the distinguishing features that
perpetuate themselves in propagation from seed. Thus, no matter what
variations occur in the individuals or the species, if they spring from
the seed of one and the same plant, they are accidental variations and
not such as to distinguish a species ... Animals likewise that differ
specifically preserve their distinct species permanently; one species
never springs from the seed of another nor vice versa.
Carl Linnaeus created the binomial system for naming species.
In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences. He established the idea of a taxonomichierarchy of classification based upon observable characteristics and intended to reflect natural relationships.
At the time, however, it was still widely believed that there was no
organic connection between species, no matter how similar they appeared.
This view was influenced by European scholarly and religious education,
which held that the categories of life are dictated by God, forming an Aristotelian hierarchy, the scala naturae or great chain of being. However, whether or not it was supposed to be fixed, the scala (a ladder) inherently implied the possibility of climbing.
Mutability
In
viewing evidence of hybridisation, Linnaeus recognised that species
were not fixed and could change; he did not consider that new species
could emerge and maintained a view of divinely fixed species that may
alter through processes of hybridisation or acclimatisation.
By the 19th century, naturalists understood that species could change
form over time, and that the history of the planet provided enough time
for major changes. Jean-Baptiste Lamarck, in his 1809 Zoological Philosophy, described the transmutation of species, proposing that a species could change over time, in a radical departure from Aristotelian thinking.
In 1859, Charles Darwin and Alfred Russel Wallace provided a compelling account of evolution and the formation of new species. Darwin argued that it was populations that evolved, not individuals, by natural selection from naturally occurring variation among individuals.
This required a new definition of species. Darwin concluded that
species are what they appear to be: ideas, provisionally useful for
naming groups of interacting individuals, writing:
I look at the term species as one
arbitrarily given for the sake of convenience to a set of individuals
closely resembling each other ... It does not essentially differ from
the word variety, which is given to less distinct and more fluctuating
forms. The term variety, again, in comparison with mere individual
differences, is also applied arbitrarily, and for convenience sake.
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